[llvm-dev] Bundling MachineInstr instructions before register allocation seems to always give errors

[Matthias Braun via llvm-dev]

In theory it should be possible. In practice you will run into issues because nobody is really doing that right now so some things like liveness updates are semi-broken. I am currently working on some patches myself for bundling some things pre-ra myself. I should put up some fixes for the infrastructure soon(ish).

- Matthias

> On Feb 23, 2017, at 6:45 AM, Alex Susu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>  Hello.
>    I am having difficulties to bundle MachineInstr instructions, before register allocation (RA).
>    More exactly, I registered a simple pass in addPreRegAlloc() that is trying to create bundles.
> 
>    I see that it is written at http://llvm.org/docs/CodeGenerator.html#machineinstr-bundles:
>      <<Packing / bundling of MachineInstr’s should be done as part of the register
>            allocation super-pass.
>        More specifically, the pass which determines what MIs should be bundled
>            together must be done after code generator exits SSA form (i.e.
>                after two-address pass, PHI elimination, and copy coalescing).
>        Bundles should only be finalized (i.e. adding BUNDLE MIs and input and
>            output register MachineOperands) after virtual registers have been
>            rewritten into physical registers.
>        This requirement eliminates the need to add virtual register operands
>            to BUNDLE instructions which would effectively double the virtual
>            register def and use lists.>>
> 
>    I presume this is the reason why I'm getting always errors after creating bundles in the pass registered in addPreRegAlloc().
>    More exactly, my pass seems to be invoked before the "Eliminate PHI nodes for register allocation" pass.
> 
>    If however, there is a simple possibility to bundle MachineInstr before RA, please let me know.
> 
>  Thank you,
>    Alex
> 
> PS: Is there a possibility to bundle MachineInstr instructions before RA and use llvm::finalizeBundle() on these bundles in a pass invoked after RA?
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